First appearances can be deceptive when investigating the root cause of an incident. Root cause analysis (RCA) can identify hidden underlying faults that, when corrected, will prevent reoccurrences. This article reviews an example where an RCA of a fire and explosion was able to identify hidden deficiencies in the plant's control systems, in which modifications had not been properly integrated.

The "root cause" of any incident is a factor which, if removed, would prevent the incident from occurring. A root cause analysis (RCA) is a systematic and structured problem solving technique to identify the root cause and prevent a recurrence of the incident. RCA is often used in the investigation of incidents at petroleum and chemical facilities.

Following an explosion and fire at a decades-old chemical facility, Baker & O'Brien was engaged to assist in an RCA of the incident. Two new electrostatic precipitators (filtration devices used to remove fine particles from the gas streams) had recently been installed, and initially it was thought that these were at fault, since an operator first saw flames emanating from one of the precipitators. However, moments after first seeing the flames, a series of explosions occurred in a process vessel upstream of the precipitators. An emergency plant shutdown was implemented and the explosions ceased.

Following a joint RCA conducted by Baker & O'Brien and the plant staff, it was determined that the root cause was actually related to the plant's control system. Over the years, and as a result of numerous process upgrades, the control system had become a patchwork of poorly integrated systems which did not work well together. The plant's control system did not adequately detect and inform the operators that flammable vapors had escaped from the upstream process vessel and made their way to the precipitators where they ignited. These flames then backed up through the system to the process vessel, resulting in the observed explosions at that location.

Apparently, an insufficiently-thorough design review had been conducted as part of the company's Management of Change (MoC) process when successive plant control system upgrades were performed. As a result of this finding, the plant staff conducted a full design review of the control systems. Prior to restarting months later, the entire control system was comprehensively upgraded.